Polymer composite materials selected and qualified for various applications, such as with primary structure applications in the manufacture of aircraft, are evaluated for two key mechanical properties: compression-after-impact (CAI) strength and hot-wet compression strength, and more specifically open-hole-compression (OHC) strength. However, the means for increasing a composite material's CAI strength and hot-wet OHC strength have typically been counterproductive to each other. More specifically, traditional particulate interlayer toughening methods using elastomeric or thermoplastic-based polymer particles have been effective for increasing a composite's CAI strength, but not generally effective for simultaneously increasing hot-wet compression strength (e.g., hot-wet OHC) properties and, more typically, result in a tradeoff relationship with one another.
Conventional methods utilized to increase the hot-wet compression strength properties of a polymer composite have usually involved increasing the resin matrix crosslink density to increase the elastic modulus of the resin or by reducing the water absorption characteristics of the matrix by proper formulation of the resin's specific chemistry. Efforts associated with increasing the matrix crosslink density to increase hot-wet compression strength typically result in a composite having reduced CAI properties.
Accordingly, it would be highly desirable to provide a polymer composite material having an interlayer structure which significantly enhances the toughness of the interlayer material, and thereby increase its CAI strength, without the negative feature of degrading the hot-wet compression strength of the interlayer.
In the interest of toughening the composite matrix interlayer sufficiently to improve its CAI strength, it will be appreciated that shape memory alloys (SMAs) are known to have unique, “super elastic” properties. One common, commercially available SMA is Nitinol®), a titanium-nickel alloy. This particular alloy, as well as other SMA materials, are able to undergo an atomic phase change from a higher modulus, austenitic phase when at a zero stress state, to a “softer,” lower modulus, martensitic phase upon the application of a load or stress. Once the load or stress is eliminated, the alloy is able to revert to its original, stress-free, higher modulus austenitic state. In the process of absorbing the energy from the induced stress, the metal temporarily deforms similar to an elastomer. This stress-induced phase change for Nitinol® alloy is reversible and repeatable without permanent deformation of the metal up to approximately 8-10% strain levels. Nitinol® alloy is further able to absorb (i.e., store) five times the energy of steel and roughly three times the energy of titanium. A comparison of the Nitinol® (NITI) alloy's superior ability to absorb energy relative to other materials is shown below:
Maximum SpringbackMaterialStrain*Stored EnergySteel0.8% 8 Joules/ccTitanium1.7%14 Joules/ccNitinol ®10.0%42 Joules/cc*maximum reversible springback without permanent deformation of strain-offset. 
In view of the foregoing, it would be highly desirable to provide a polymer composite structure having a matrix interlayer which provides the superelastic properties of a SMA, but which does not significantly add to the weight of the overall structure, and also which does not negatively affect the hot-wet compression strength of the matrix interlayer.